The present invention relates to semiconductor processing. More specifically, the invention relates to methods and apparatus for uniformly forming various types of films. Embodiments of the present invention are particularly useful to deposit thin films, including metal-containing films such as a tungsten (W) film or a tungsten silicide (WSi.sub.x) film, undoped dielectric films such as undoped silicate glass (USG) films, doped dielectric films such as borophosphosilicate glass (BPSG), phosphosilicate glass (PSG) or borophosphosilicate glass (BSG) films, and other films. In addition, other embodiments of the present invention may also be used for economically and efficiently manufacturing semiconductor devices from processing substrates of various diameters.
One of the primary steps in fabricating modern semiconductor devices is forming a film on a semiconductor substrate. As is well known, such a film can be deposited by chemical vapor deposition (CVD). In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a conventional plasma process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film. In general, reaction rates in thermal and plasma processes may be controlled by controlling one or more of the following: temperature, pressure, and reactant gas flow rate.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two-year/half-size rule (often called "Moore's Law") which means that the number of devices which will fit on a chip doubles every two years. Today's wafer fabrication plants are routinely producing 0.35 .mu.m and even 0.25 .mu.m feature size devices, and tomorrow's plants soon will be producing devices having even smaller feature sizes. It is also important that generation of particles in processing chambers be avoided to reduce contamination of substrates that reduces the yield of good devices. It is increasingly important that deposited thin films be of uniform thickness across the substrate for device uniformity.
In substrate processing apparatus, problems may arise because the film deposits not only on the topside of the substrate, as desired, but also undesirably on the edge surfaces and backside of the substrate. Because the deposited film, for example tungsten, may not adhere to the edge and backside surfaces of the silicon substrate, the material deposited on the edge surfaces and backside of the substrate tends to flake off and contaminate the processing chamber. Also, the uneven surface of the substrate due to unwanted deposition on the edge surfaces and backside may undesirably result in, for example, peeling problems in a chemical mechanical polishing (CMP) step or other problems in other subsequent device fabrication steps. For example, these problems have been encountered in CVD chambers used to deposit metals such as tungsten using tungsten hexafluoride (WF.sub.6), a highly volatile gas. Edge rings have been used to cover or protect the periphery of the substrate during deposition, thereby preventing the deposition gases from reaching the edge and backside surfaces of the wafer. However, due to the volatility of WF.sub.6, for example, the use of a purge gas directed behind or at the edge of the substrate behind the edge ring has been tried. The purge gas exerts a positive pressure that reduces the chance that the processing gas will reach these edge and backside surfaces.
Despite the use of edge rings and purge gases, the deposition of thin films, including a metal film such as tungsten, by CVD may not be as uniform as desired for some applications. With conventional CVD apparatus, uniformity issues with the deposited film may arise due to non-uniform pressures in the purge gas channels. In particular, some conventional CVD apparatus utilize a heater assembly 1, such as shown in FIGS. 1A and 1B, that includes a top metal block 2 to support a substrate 16 thereon, a bottom metal block 3 having a single turn coil resistance heater element 4 embedded therein, and a metal edge ring 5. Edge ring 5 is a separate metal ring that is welded (along its perimeter as indicated by arrows "w") onto top metal block 2. The coil 4 is in contact with the bulk of heater assembly 1 in order to provide uniform heating of top metal block 2 and to uniformly heat substrate 16 mounted thereon. Further, substrate 16 that is vacuum-mounted on such top metal block 2 may be rapidly heated uniformly using heat assembly 1. In such conventional CVD apparatus, edge ring 5 and top metal block 2 form an annular slot 6 through which purge gases from bottom metal block 3 flow between the edge ring and the periphery of the substrate along the edge of top metal block 2 to prevent undesired edge and backside surface deposition on the substrate 16. Various purge channels 7 in a complex linear pattern are formed in the bottom metal block 3 proximate to top metal block 2, as seen in FIG. 1B which illustrates a top view (shown without substrate 16) of heater assembly 1. The purge gas enters via a vertical purge inlet passage 8 through the bottom of the bottom metal block 3 to the center of a main, straight horizontal purge channel 7 along the diameter of the bottom metal block 3 and also to other purge channels 7. Specifically, multiple horizontal purge branch channels, perpendicular to the main purge channel, branch out from the main purge channel and lead to an annular purge channel 9 in bottom metal block 3. From annular purge channel 9, the purge gas flows through slot 6 between edge ring 5 and along the edge of substrate 16 to prevent undesired deposition on substrate 16. Purge gas traveling along these various purge channels 7 often experience different effective pressures at different locations depending on which particular channel 7 the purge gas traveled. The different effective pressures of the purge gas at different locations may result in non-uniformity in the deposited film. Further, such conventional CVD apparatus may experience worsened uniformity problems over time. In such apparatus, the top part of edge ring 5 is bulkier than the narrow welded bottom part of edge ring 5 near bottom metal block 3. For processing temperatures reaching about 400.degree. C., the heavier top part of edge ring 5 tends to warp radially outward away from top metal block 2, which may result in non-uniform purge gas flow due to distortion of slot 6. The edge ring warpage thus results in additional film uniformity issues due to the potentially uneven flow of purge gases along the unevenly warped edge ring. Once the edge ring has begun to warp, the edge ring, which does not return back to its original shape, becomes unusable due to the resulting non-uniform purge gas flow and must be replaced.
In order to more economically and efficiently produce such devices, manufacturers desire to fabricate the devices using increasingly larger diameter substrates, such as 12-inch (or 300-mm) diameter and even larger substrates. Processing larger diameter substrates requires substrate processing equipment to not only physically accommodate such larger substrates but also still meet stringent requirements (for example, adequate substrate heating ability and uniform film deposition) for fabricating high integration devices. However, designing such substrate processing equipment to perform adequately for high performance applications may be an expensive endeavor. For example, the above-described heater assembly 1 having a single turn coil heater element 4 becomes inadequate to provide the heating capability needed to heat larger diameter substrates to deposit uniform films thereon. The above-described heater assembly 1 having metal edge ring 5 welded onto bottom metal block 3 having complex purge channels 7 therein also is difficult to scale for larger diameter substrates without performing extensive experimentation and optimization for uniform purge gas flow and for thin film deposition required for increasingly integrated devices. Further, even if the above-described heater assembly 1 were to be scaled up for larger diameter substrates, the larger diameter of the heater would require an even larger edge ring, which being thin and larger, would be even more likely to warp easily. To avoid costly redesign efforts and warpage problems for heaters for each subsequently larger diameter substrate, it is desirable to provide substrate processing equipment which is scaleable in design for processing different diameter substrates and/or is operable without regard for the diameter of the substrate processed. Such flexibility in substrate processing equipment design can result in greater cost savings and efficiency in substrate processing, especially as processing equipment evolves for larger diameter substrates.
In light of the above, an improved heater assembly that has a simplified design scaleable for equipment processing different diameter substrates is needed to efficiently and economically process substrates to meet stringent film requirements such as film uniformity.